1. Field of the Invention
The present invention relates generally to a method and apparatus for checking the position of leads on an electrical component.
2. Description of the Related Art
In the automatic equipping of printed circuit boards or ceramic substrates with SMD components, the individual components are removed from a magazine or a feeding device by means of a equipping head and are then positioned in a predetermined position on the printed circuit board or the ceramic substrate. Since the components exhibit, in the magazine or in the collection position of a feeding device, a position tolerance of approximately 1 mm, but must be positioned on the printed circuit board or the ceramic substrate with high accuracy, an automatic determination of position and correction are necessary. Furthermore, especially in the case of SMD components with a large number of pins, the separation and the coplanarity of the leads must be checked. The determination of position and the checking of separation and coplanarity should in this case take as little time as possible, in order to permit a high degree of equipping performance of the automatic equipping system. Increasing component dimensions of up to 70.times.70 mm, decreasing separation of the leads of down to 0.3 mm and stringent requirements placed upon the speed of equipping create considerable difficulties in achieving the mentioned objects.
Known arrangements for determining the position and checking the separation of the leads of components obtain the image of the component or sections of the component, by means of an objective lens, onto a planar CCD camera and ascertain, using digital image processing, the position of the component leads and the separation of the leads. With such arrangements, a coplanarity check is not possible.
In other known arrangements for determining the position and checking the separation of the leads of components, the components are placed onto an optically transparent plate, on which a shadow of the illuminated leads is generated. The shadow edges of the leads are then imaged from below by means of an objective lens onto a planar CCD camera, so that in this case also, again, the position of the component leads and the separation of the leads can be ascertained using digital image processing. A coplanarity check is made possible using these arrangements in that the leads are successively illuminated from two different directions (cf. European Patent Application 0 425 722 and PCT Published Application 91/15104).
As a result of the increasing component size, as well as the more stringent requirements imposed on accuracy and the required short determination times, significant limits are placed on a use of the known arrangements for determining the position, checking the separation and possibly checking the coplanarity of the leads of components, since:
In the case of a dimensioning of the components of for example 70.times.70 mm, a CCD camera having 4000.times.4000 pixels would be necessary. PA1 The read-out time of the corresponding information, at approximately 1 s, lasts far too long and is far too great for rapid processing. PA1 It is necessary to provide objective lenses which while having a field of view of 70.times.70 mm exhibit a freedom from distortion of 5 micrometers. Such objectives can be produced only at high cost and are heavy and large. PA1 On account of the weight and size of the illuminating device and of the objective lens, the known arrangements for determining the position and checking the separation can scarcely be integrated into the equipping head of an automatic equipping system; in this case, such an integration is definitely to be ruled out with respect to a certain size of the components. The solution which is optimal with regard to the equipping time, involving a determination of position and checking during the shortest equipping path between component removal and component positioning, cannot then be implemented.
German Patent Application 35 46 216 discloses an arrangement for determining the position of components, in which arrangement the equipping head brings up an image sensor, such as for example a television camera, and the image sensor then records an image of the component at the suction pipette of the equipping head. According to a variant, it is also to be possible to fit the image sensor on the equipping head. Having regard to the overall volume and the weight of the image sensor, this variant could however at best be feasible for the equipping of small components.
German Patent Application 33 40 084 discloses a device for the equipping of printed circuit boards or ceramic substrates with components, in which device the equipping head brings up a position pickup device, in which a relative displacement of the component in relation to a theoretical position related to the positioning axis of the equipping head is ascertained. The position pickup device includes a frame, which is provided along one frame side with a series of mutually adjacent, radiation-emitting elements and along the opposite frame side with a series of mutually adjacent elements receiving the mentioned radiation. If now a component held at the suction pipette of the equipping head is transported into the frame, then the centrally disposed receiving elements lie in the shadow of the component. The ratio of the left-hand and the right-hand receiving elements, which receive a beam, forms a measure of the eccentricity of the component in the corresponding direction.